This invention relates to the art of chemically treating an aluminum surface. It relates to improving the corrosion resistance and paint adhesion of an aluminum surface for manufacturing cans, structural materials for buildings, automobiles and electric goods. More specifically, it relates to the art of treating the surface of an aluminum can to improve both the corrosion resistance and the adhesion of an organic finish subsequently applied to the treated surface. This invention also concerns a process for accomplishing the foregoing results with an aqueous solution which has a less detrimental effect upon the environment than conventional treating solutions because it does not require the presence of chromium. The invention further relates to a composition of the above type which does not impart a noticeable color to the treated surface.
The term "aluminum" as used herein is meant to include alloys of at least 90 percent aluminum which are commonly employed in can manufacture. Such alloys may contain elements such as magnesium, manganese and zinc, for example. 3000, 5000 and 6000-type aluminums are suitable examples.
In the processing of aluminum cans after forming, the following procedure is typical:
1. wash with warm water; PA1 2. clean, usually with an acid-type cleaner; PA1 3. water rinse; PA1 4. apply treatment chemical; PA1 5. water rinse; PA1 6. deionized water rinse; PA1 7. dry; PA1 8. apply decorative organic finish to the can exterior; PA1 9. cure decorative organic finih at elevated temperature; PA1 10. apply interior sanitary lacquer; and PA1 11. cure interior sanitary lacquer. PA1 1. corrosion resistance; PA1 2. paint adhesion; PA1 3. coating color and gloss; PA1 4. required reaction time.
As used herein, the term "organic finish" includes all organic films applied after step 4, for example: base coat, ink, paint, over-varnish and sanitary lacquer.
For step 4), standard practice is to employ an aqueous solution containing from one-half to one weight percent of a mixture of hexavalent chromium, phosphoric acid, and fluoride. Such treating solutions have produced satisfactory quality in terms of both corrosion resistance and paint adhesion. However, the chromium component is environmentally objectionable, and its use therefore entails additional recovery expense. It would, therefore, be highly desirable to be able to use a treating solution which would produce acceptable results which did not at the same time create the environmental problem of solutions containing chromium.
The following characteristics of an applied coating must be examined to determine its acceptability for use in aluminum can treatment:
Testing for corrosion resistance and paint adhesion is performed as described prior to the examples. Coating color and gloss are rated visually. The color and gloss are important because the bottom of the can is not normally painted and because very often the decorative organic finish is applied to only a portion of the can surface allowing a portion of the aluminum surface to show through. Therefore, if the coating discolors the surface, the desired effect is not obtained. Finally, current manufacturing designs allow only very short reaction times (e.g., 20 seconds). Treating compositions which produce satisfactory results with respect to corrosion, adhesion and color must also do so within the permissible contact time.
The use of tannins in connection with metal treating has been suggested by the prior art. U.S. Pat. No. 2,502,441 discloses an alkali metal phosphatizing solution containing a two-component accelerator which may be used for the treatment of iron and steel surfaces and also possibly for other metals such as aluminum. The accelerator portion of the composition contains either a molybdenum or tungsten compound and a phenolic substance such as a tannin. The patentee notes, however, that if the alkali metal plus tannin is used without the molybdenum compound, deposition of a coating seems to be completely inhibited. Further, when either tungsten or molybdenum compounds are employed with fluoride to treat aluminum, objectionable discoloration of the surface occurs. U.S. Pat. No. 2,854,368 teaches the use of a phosphoric acid solution containing a tannin for the treatment of iron or steel and also possibly for other metals such as aluminum. The most dilute solution suggested by the patentee is one containing one mole of phosphoric acid and one weight percent tannin. When this solution was substituted for that of the present invention, completely unsatisfactory adhesions were obtained.